This invention relates to the coating of linear material such as sheet, strip, and especially wire, with metal coatings in a molten metal coating bath. More particularly the invention relates to the combined use of protective atmospheres and gas wiping in treating linear material issuing from a molten metal coating bath in order to establish an accurate thickness of coating on the surface of the linear material.
Metallic linear material such as sheet, strip and wire has been economically coated for many years by passing the linear material through a bath of molten metal such as molten zinc or aluminum. Usually the linear material has been a ferrous material such as steel or the like. The outer coating of aluminum or zinc or sometimes other metals or alloys such as tin or terne (an alloy of lead with up to 25% tin) provides corrosion resistance to the underlying ferrous metal.
Linear material passing from a molten metal coating bath usually does not have a satisfactory layer of molten coating metal on its surface. The molten metal coating is invariably either too thick, too uneven, or both, or has some other defect which would prevent the molten metal from solidifying into a uniform metal coating upon the substrate metal. As a consequence, it has been customary to wipe the coating in some manner after the linear material leaves the molten coating bath in order to smooth and/or reduce the weight, or thickness, of the coating. Various wiping devices have been used to wipe the coating while it is still molten including soft wipers such as asbestos wipers and the like, rigid wipers such as rolls and scrapers and occasionally semi-rigid wipers composed of layers of various materials such as a charcoal or gravel through which the coated linear material passes. More recently gas wipers, or gas doctors, have been used to blow a gas such as air, steam or some inert or reducing gas forcibly against the surface of the molten metal coated linear material to remove excess metal and smooth the coating of molten metal.
In order to attain good adherence of the coating metal to the substrate metal it is necessary for the surface of the substrate to be clean prior to passage through the molten coating bath. The linear material must, therefore, be cleaned prior to being coated to provide a suitable clean, active substrate surface for contact with the molten coating bath. Otherwise the molten coating will frequently not adhere to the surface. Once the substrate metal is clean it must be kept active, i.e. oxide free, until it is submerged in the molten coating bath. It is therefore necessary to protect the substrate metal after cleaning either with a coating of flux or else by immersion in an inert or reducing atmosphere. Thus, ferrous linear material frequently enters the molten bath from a protective or oxygen excluding atmosphere. The protective atmosphere is composed of either an effectively inert gas or a reducing gas or gases.
Inert or reducing atmospheres have also been maintained about the linear material as it exits from the molten bath to prevent excessive or otherwise detrimental oxidation of the surface of the coating while it is still hot, both before and after the coating solidfies. The protective atmosphere is usually contained in a hood which extends to or into the surface of the molten bath.
With the more recent frequent use of gas wipers for smoothing and wiping molten coatings, the use of an inert or reducing gas to wipe the surface of the linear material has sometimes been adopted to prevent surface oxidation. In some installations, and particularly in wire wiping installations, the wiper has been enclosed in or attached to a chamber containing a protective atmosphere so that the molten coating on the wire leaving the molten coating bath is completely protected from exposure to the atmosphere until it is wiped.
The use of a non-oxidizing gas as both a wiping and a protective gas has been found to be particularly desirable in the wiping of wire material. Otherwise oxidized coating particles on the molten coating surface tend to increase the viscosity of the molten metal and result in buildup of a thick viscous oxide coating layer which seriously interferes with effective gas wiping. The small circumference of the wire allows viscous rings of oxide material to form about the wire and break through the gas barrier resulting in thick rings of coating on the wire. Coatings including such rings crack and flake when the wire is bent after solidification.
One problem which has been encountered in combined wiping and protective gas installations such as, for example, that illustrated in U.S. Pat. No. 3,707,400, which discloses a combined closed hood, which may contain an inert gas, and a wiping die, which may use an inert gas as a wiping gas, has been a tendency of the wiping die to provide very poor control of the thickness of the final coating if only the force of the wiping gas is depended upon to establish the thickness of the coating. This has been so in spite of the fact that such combined wiping and protective gas arrangements very efficiently and effectively wipe excess coating from and smooth linear material such as wire passing through the die. The exact final thickness of coating has often, however, been impossible to control without varying the parameters of the wiping die itself. In other words, while the smoothing of the coating is very effective and a large excess of coating material can be removed from the coated material, actual control of the coating thickness to any specified coating thickness by control of the wiping gas has frequently not been satisfactory. It has thus been necessary in many cases to vary the velocity of passage of the linear material through the wiping die in order to effectively control the degree of wiping of molten coating from the surface of the linear material. If the molten coating layer is too thick, it has been necessary to decrease the speed of passage of the linear material through the die orifice in order to decrease the coating layer. If the coating layer is too thin, on the other hand, it has been necessary to increase the speed of the linear material through the die orifice in order to increase the thickness. Naturally, the necessity to adjust the speed of the coating line in order to attain a desired coating weight is undesirable, because such adjustment interferes with other operational and production considerations.
It has been possible to effectively control the coating weight on linear material passing between gas wiping dies which are not associated with a closed gas hood, for example, by the use of the type of opposed gas wiping dies illustrated in U.S. Pat. No. 3,499,418 to Mayhew, merely by controlling the force of the gas blast or the distance of the gas dies from the surface of the material being wiped. However, when a closed hood has been associated with the die as shown for example in U.S. Pat. No. 3,707,400 mentioned above, effective control of the coating weight has not been found to be conveniently possible merely by varying the flow of wiping gas through the die and impinging upon the surface of the molten coating on the linear material. Instead it has been necessary, as pointed out above, to either vary the speed of the material through the die or else to vary the parameters of die, for example with respect to the length and diameter of the wiping die orifice and the like. This has meant, in effect, that if it is desired to run a coating line at a constant speed different sized dies must be used or substituted for each other each time a significant adjustement in thickness of the final coating has been desired. Such substitution of dies each time an adjustment in coating thickness must be made is obviously impractical.